With recent advances in the digital technology, digital record/playback apparatuses have become popular which are capable of recording and playing back data many times without causing degradation in image/sound quality. In such a situation, a large number of digital contents of various fields, such as video and music contents, are transmitted and distributed using recording media such as a DVD or a CD or through a communication network.
In digital recording/playing back, unlike analog recording/playing back, high quality similar to that of original data can be maintained, because data can be recorded and played back repeatedly many times without causing data to be degraded. However, the popularity of such high-quality digital recording/playing back technology results in spreading of a large number of illegally copied data, and thus serious problems in protection of copyright have arise.
To protect copyrights of digital contents from being pirated by illegal copying, it has been proposed to add copy control information for control copying to digital contents and read the copy control information when digital contents are recorded or played back thereby preventing digital contents from illegally copied.
Controlling of copying of contents can be performed in many modes depending on information sources. For example, any copying is prohibited in one mode, but in another mode, copying from original data is permitted only once and further copying from the copied data is prohibited (this mode is called copy generation management).
A typical copy generation management method is known as the CGMS (Copy Generation Management System) method.
When the CGMS method is applied to an analog video signal, copy control information (CGMS-A information) is described by 2 bits in a total of 20 bits of additional information and superimposed on a luminance signal of a video signal in a specific horizontal interval within a vertical blanking period. In the case of a NTSC video signal, the specific horizontal interval described above is the 20th effective horizontal interval. In the case of a digital video signal, 2-bit copy control information (CGMS-D information) is inserted into the digital video data, and the digital video data is transmitted together with the inserted copy control information.
The 2-bit information according to the CGMS method (hereinafter, referred to as the CGMS information) has one of the following values, depending on the copy control mode: “00” (copying is permitted freely), “10” (copying is permitted once (one-generation copying is permitted)), and “11” (copying is prohibited (unconditionally)).
In a case in which CGMS information added to image information has a value of “10”, a recording apparatus adapted to the CGMS method determines that the received video signal is permitted to be copied, and records the video signal. In this case, CGMS information rewritten into “11” is added to the recorded video signal. On the other hand, in a case in which CGMS information added to video information to be recorded has a value of “11”, the recording apparatus adapted to the CGMS method determines that the video signal is prohibited to be copied, and the recording apparatus does not record it. An IEEE1394 interface is known in the art as an interface for digital video data. It has been proposed to use CGMS information to protect copyrights of data transmitted via the IEEE1394 interface.
In addition to the copy control method using CGMS information, there are many other known methods for protect copyrights of contents. For example, in digital broadcastings performed by broadcasting stations, digital copy control descriptor is stored in serving information (SI) included in a transport stream (TS) packet by which digital data is carried. When digital data received by a receiver is recorded using a recording apparatus, generation-based copy control is performed in accordance with the digital copy control descriptor.
It has been proposed to describe copy control information in the form of a digital watermark (WM) that cannot be viewed or perceived when a content (such as video data or audio data) in which the digital watermark is embedded is played back in a normal manner. When the content is processed by an apparatus such as a receiver, a recorder, or a player, the digital watermark (WM) is detected, and generation-based copy control is performed in accordance with the detected digital watermark (WM).
Because digital watermarks can be detected and embedded only by specific devices, tampering with data is very difficult. Thus, in this technique, data is highly reliable compared with the CGMS method in which control information is added in the form of bit data.
However, a problem of this technique is that the detected level of digital watermark information greatly varies depending on the content of an information signal (such as a video or audio content) to which a digital watermark is added or various processes such as data compression, decompression, or scrambling, performed on the signal after the digital watermark was added to the content, and thus the time needed to detect the digital watermark information from the content varies depending on the content.
More specifically, in the process of detecting a digital watermark embedded in video data, although the digital watermark can be detected instantaneously in some cases, there is a possibility that the digital watermark cannot be detected in a long continuous period such as few seconds or few ten seconds depending on the video data.
FIG. 20 shows an example of a process of detecting a digital watermark embedded as copy control information in data. In this example shown in FIG. 20, a digital watermark is detected when digital data broadcasted from a station is received or the received digital data is recorded. In digital broadcasts provided by broadcasting stations, various programs are provided one after another, and various commercial messages provided by sponsors are inserted between programs. Thus, copyright of contents provided via broadcasting varies frequently and sequentially.
In a case in which a digital watermark is embedded as copy control information, a digital watermark functioning as copy control information indicating that copying should be controlled in the Copy Free mode, the Copy Once mode, or the Never Copy mode is embedded in each content of programs and commercial messages.
In the example shown in FIG. 20, a digital watermark functioning as copy control information indicating that copying should be controlled in the Copy Once mode is embedded in a content provided during a period A, and a digital watermark functioning as copy control information indicating that copying should be controlled in the Copy Free mode is embedded in a content provided during a period B. In a receiver or a record/playback apparatus, a digital watermark is detected from a content and the content is recorded into a digital device.
When a content having copy control information indicating that copying should be controlled in the Copy Once mode is digitally recorded by a digital recording apparatus adapted to recording (playing back) of contents according to copy control information, if copy control information indicating that copying should be controlled in the Copy Once mode is detected from the content, the digital recording apparatus rewrites the copy control information into No More Copy and records the content together with the rewritten copy control information. Thus, if the content recorded by the digital recording apparatus is tried to be further recorded into another digital device, copying is prevented by the copy control information set to No More Copy.
Detection of a digital watermark from a content is performed repeatedly for each image frame. However, as described earlier, the levels of detected digital watermarks, that is, the detected levels, vary depending on the condition of the video data, and the digital watermarks cannot be accurately detected unless the detected levels are higher than a predetermined threshold (Th). Therefore, the digital watermarks are read only when the detected levels are higher than the threshold (Th). If a digital watermark includes copy control information, copying is controlled in accordance with the detected copy control information. More specifically, in accordance with the control information, deciding whether to permit the operation of recording the content onto a recording medium is performed, and the copy control information is rewritten, for example, from Copy Once to No More Copy.
In FIG. 20, detection timing arrows (denoted by a, b, c, d, e, f, and g) indicate times at which digital watermarks having a level higher than the threshold (Th) is successfully detected. If a digital watermark is detected, the process performed thereafter obeys the detected information at least over a following period with a predetermined length (T). In the case of the content A, because the digital watermark including the copy control information indicating that copying should be controlled in the Copy Once mode is embedded in the content A, copying is controlled in accordance with the copy control information indicating that copying should be controlled in the Copy Once mode. If no digital watermark is detected in a period with the predetermined length (T), the detected information is determined to be “undefined”, that is, the process is performed assuming that that no control information is included in the content. There is no obligate rule to be applied to the “undefined” case, and the process for an “undefined” period may be performed, for example, in a similar manner as is performed when the content includes copy control information indicating that copying should be controlled in the Copy Free mode.
In FIG. 20, at a time denoted by a detection timing arrow c, digital watermark information is detected from the content A, and the process is performed in accordance with the detected copy control information indicating Copy Once. Thereafter, at a time (denoted by an arrow d) within a following period of T, a digital watermark including copy control information indicating that copying should be controlled in the Copy Once mode is detected. After that, if the content is switched from A to B, no digital watermark is detected for a while. In this case, the receiver or the record/playback apparatus that is processing the content obeys not to the copy control information (Copy Free) of the content B but to the copy control information (Copy Once) of the content A in an initial period (denoted by “wrong decision” in FIG. 20).
When the digital recording apparatus is recording the content, if the content being recorded is switched from the content A including copy control information indicating Copy Once to the content B including copy control information indicating that copying should be controlled in the Copy Free mode, and if the initial part of the content B (in the “wrong decision” period in FIG. 20) is processed in accordance with the copying control information (Copy Once) of the content A as shown in FIG. 20, the initial part of the content B is recorded together with copy control information rewritten to No More Copy indicating that no more copying is permitted in accordance with the copy control information (Copy Once) of the content A, although the initial part of the content A is actually permitted to be freely copied. Thus, there occurs a problem that the initial part of the content B cannot be further copied, although that part should be permitted to be copied.